In Ansible, a handler is just like any other task but only runs when called or notified. It takes action when a change has been made on the managed host. Handlers are used in initiating a secondary change such as starting or restarting a service after installation or even reloading a service after some modifications have been made in the configuration files.  In this guide, we will shed more light on Ansible handlers. We will learn how to use handlers in ansible playbook.

Ansible playbook file with a handler

To better understand how Handlers work, we will take an example of a playbook file – install_apache.yml – that installs the Apache webserver and later restarts the Apache service. In the example below, the handler is notified to restart the Apache service soon after installation. This is achieved using the notify module as shown. Note that the ‘notify’ name should coincide with the handler name as pointed out, otherwise you will encounter errors in your playbook file.

--- - hosts: staging   name: Install   become: yes   tasks:           - name: Install Apache2 on  Ubuntu server             apt:                     name: apache2                     state: present                     update_cache: yes            notify:                   - Restart apache2  handlers:           - name: Restart apache2             service:                     name:  apache2                     state: restarted

Ansible-Playbook-with-Handlers

Now let’s run the playbook file.

$ ansible-playbook /etc/ansible/install_apache.yml -K

From the output, you can see the Handler being executed right after the task.

Ansible-Playbook-Execution-Handlers

Multiple tasks with multiple handlers

Additionally, we can have several tasks calling multiple handlers. Consider the playbook file below.

Here are have 2 tasks to run:

  • Installing Apache webserver
  • Allowing HTTP traffic on the UFW firewall.

After the tasks are successfully executed, I have called each of the handlers with the ‘notify’ module as shown below. The first handler restarts Apache and the second one reloads the UFW firewall.

--- - hosts: staging   name: Install   become: yes   tasks:         - name: Install Apache2 on  Ubuntu server           apt:                   name: apache2                   state: present                   update_cache: yes         - name: Allow HTTP traffic on UFW firewall           ufw:                   rule: allow                   port: http                   proto: tcp           notify:                   - Restart apache2                   - Reload ufw firewall   handlers:           - name: Restart apache2             service:                     name:  apache2                     state: restarted           - name: Reload ufw firewall             ufw:                     state: enabled

Multiple-Handlers-Ansible-Playbook

When the playbook file is executed,  both handlers are executed by Ansible right after Apache is installed and HTTP traffic is allowed on the firewall.

The secondary actions executed by the handlers here are:

  • Restarting Apache
  • Enabling and reloading the firewall for the changes made to be effected.

Ansible-Playbook-Execution-Multiple-Handlers

Conclusion

As you have seen, handlers are just like regular tasks, only that they are referenced using a globally unique module called ‘notify’. If a handler is no notified, it fails to run. Remember that all handlers run after all the tasks have been completed.

Also ReadHow to Create Ansible Roles and Use them in Playbook

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